Use the Highlight, Underline and Strikethrough tools to mark up text. With the tool active, select the text you want to mark; the markup is applied as soon as you release the selection.
Use the Add text tool to place a new text box anywhere on the page, and the Sticky note tool to attach a note. For both, click the spot on the page where you want it to appear.
Like every change in Seben, each markup is saved as a new version, so you can undo it.